Electric protective equipment



Nov. 25, 1958 T. R. coGGEsHALI. l-:TAL

ELECTRIC PROTECTIVE EQUIl-MENT Original Filed May 6, 1953 Inventors:Thel lwel I F?. C022 esl'w all,

H er m an Ban 5,

bO'r`T1e1.

/f/ l iff/ H///// y ing) ELECTRIC PROTECHVE EQUIPMENT Thellwell R.Coggeshall, .Cynwyd, and Herman Bany,

Lansdowne, Pa., assignors to,General Electric Company, a corporation ofNew York Original application May 6, 1953, Serial No. 353,358,

now Patent No. 2,304,576, dated August27, 1957. Divided and thisapplication November 30, 1956, Serial No. 625,350

8 `Claims. (Cl. 336-173) This application is a division of ourapplication Serial No.` 353,358, now Patent No. 2,804,576, tiled May 6,195.3, and assigned to the assignee of the present invention.

The present invention relates to a current transformer construction and,:more particularly, to a current transformer construction which isespecially adapted for use in an overlapping diiferential protectivesystem.

To facilitate an understanding of our invention, we have illustrated anddescribed the invention as being utilized in differential protectivecircuits having current transformer windings which are directlyconnected. However, it will be apparent to those versed in theprotective relaying art, that the invention is equally applicable toprotective equipment in which the windings are interrelated byanddesired signal transmission channel over which information may beconveyed conductively or electromagnetically to effect relay operation.Examples of such protective systems are the .conventional wire-pilot andcarrier-current pilot types of relaying equipment. Since all of thesetypes of relaying equipment operate on a differential principle, theyare referred to generically hereinafter as being of the differentialprotective type.

Some of the problems solved by our invention should become apparent fromthe following description of certain limitations of conventionaldifferential protective equipment. For example, for sensing a fault in agiven zone of an electric power circuit, it is conventional to mount apair of current transformer windings at opposite ends of the zone and toconnect the windings in a differential protective circuit. A relaysuitably connected in such a differential circuit will operate inresponse to a fault occurring inside this protective Zone but willremain inactive if the fault is external to this protected zone. Whereit is desired to provide a pair of such protected Zones in the powercircuit, a pair of such differential protective circuits are used. Thecurrent transformer windings of these circuits are usually arranged inlocations such that the pair of protected zones overlap, i. e., oneprotected Zone extends into the other protected Zone so as to form aZone of overlap common to both protected zones. With such an overlappingprotective system, faults occurring within the zone of overlap aresensed by both differential protective circuits, whereas those in only asingle protective zone are sensed only by the single differentialprotective circuit protecting that particular zone. Thus, such anoverlapping system can distinguish between faults within the zone ofoverlap, and faults external thereto; and can further distinguishbetween those external faults occurring at one side of the overlap zoneand those external faults occurring at the other side of said overlapZone.

Where it is desired to distinguish between faults occurring within acircuit breaker and those external thereto and to further distinguishbetween those external faults occurring at one side of the breaker andthose external faults occurring at the other side of the breaker, it hasvCircuit breaker. `the tank-type .circuit breaker having bushing-typecur- ICC been proposed heretofore to arrange a pair of currenttransformer windings `of such an overlapping system at oppositeterminalsof the circuit breaker so that the zone of overlapvisco-extensive with the internal circuit of the Such anlarrangement isexemplied by rent transformers mounted on each of its terminal bushings.ln such an arrangement, substantially all faults occurring in theinternal circuitof Vthe circuit breaker will. be faults .intheoverlapzone, which the system can distinguish from faults ,external to thebreaker, the latter of which are .faults located in only one protectedzone, i. e., beyond either end ofthe overlap Zone. Some circuitbreakers, however, have no bushings, and thus do not lend themselves totheuse of current transformer windings atopposite terminals, orbushings. From the standpoint of economy, it is sometimes desirable insuch breakers to mount the pair of windings at points remote from thefbreaker '.terminals. ln such an arrangement, the zone of overlap is notco-extensive with the Vinternal circuit of the `circuit breaker, andhence, unless vcertain modications are made in the protective system,

incorrect operation `will take place for certain fault conditions, e.g., the -occurrence of faults internal to the circuit breaker butexternal to the overlap Zone.

It is, therefore an object of this invention to provide a currenttransformer construction which makes it possi- .ble to mount the currenttransformer Awindings of an overlapping differential relaypr-otectivesystem at points .remote from the terminals of a circuitybreaker or the like without atfectinglthe ability of the transformerwindings to distinguish between faults in the internal circuit ,of thecircuit breaker and those faults external to said internal circuit.

It is a further object ofy this invention to shield the currenttransformer windings in such a manner that they may be mounted injuxtaposed relationship on a housing for acircuit breaker or the like bya sturdy and inexpensive mounting and yet are adapted in thisrelationship to accurately distinguish between faults occurring Awithinsaid housing and those external thereto.

It is still another object of this invention to construct and shield thecurrent transformer windings in such a manner that alternate breakdownpaths to ground with respectto the transformers are provided, one pathfavoring internal fault currents and the other path favoring external=fault currents in such a manner that the current transformers mayaccurately distinguish between internal and external faults. The termfault current, as used in this application, denotes that current whichows to ground from a breakdown point in the power circuit.

In carrying out our invention in one form, we provide a pair ofjuxtaposed current transformer windings which are adapted to be mounted4on the housing-of a circuit `breaker or the like at points remote fromthe terminals of the breaker. The windings are encased by conductiveshielding structure which is so constructed that fault currents externaltothe housing will takeV one breakdown path to ground with respect tothe windings and fault currents internal lto the housing will take adifferent breakdown path to ground with respect to the windings. The`latter path to ground is inductively linked with one of the windingsand extends between the two windings, .whereas the former path to groundis external to the windings and is essentially independent of inductivelinkage with said windings, whereby to cause said current transformerwindings to accurately distinguish between internal and-external faultcurrents so as to cause proper relaying of an associated differentialprotective system.

The invention will ybebetter understood by considering the followingdescription taken in connection with the may be of any conventionaltype.

accompanying drawing, and its scope will be pointed out in the appendedclaims.

The single iigureof the drawing is a view, partly in schematic form andpartly in section, showing an electrical system embodying the invention.

The electrical system shown in the drawing may be designed for eithersingle phase or polyphase operation, but for the purposes of simplicity,portions of the system are illustrated by means of a conventional oneline diagram.

A pair of electrical conductors which, for illustrative purposes, may bedesigned bus sections 1 and 2 are electrically interconnected through acircuit breaker 3 which may be termed the bus-tie circuit breaker.Connected to bus section 1 are electrical circuits 4 and 5, andconnected to bus section 2 are electrical circuits 6 and 7. Theseelectrical circuits may be feeder circuits for supplying electricalenergy to a bus section, or may be distribution circuits for supplyingelectrical energy from a bus section to a load. The circuits 4, 5, 6 and7 are respectively connected to associated bus sections 1 or 2 throughcircuit breakers 8, 8a, 8b and 8c. These circuit breakers are shown onlyschematically in the drawing since they Since all of these circuitbreakers may be substantially identical, it is considered necessary todescribe only a single one. More particularly, circuit breaker 8,located at one side of the bus-tie breaker 3, includes a pair of fixedcontacts 9 adapted to be bridged by a movable bridging contact 10 whichis biased toward open position and is latched in the closed positionshown by a latching mechanism 11. Latch 12 of the latching mechanism isbiased toward latching position and is motivated to the tripped positionby means of an electromagnetic device or solenoid 13 which may deriveenergy from a tripping source which preferably is a direct currentsource indicated at 14. For simplicity, the breakers 8b and 8c on theopposite side of the bus-tie breaker 3 are shown as having their tripmechanism operable from a separate tripping source 14', but it isobvious that all of the breaker trip mechanisms could be operated from asingle source.

The bus-tie breaker 3 is a high voltage impulse type of breaker havingterminal portions 15 and 16 respectively connected to bus sections 1 and2. Terminal portion 16 has a relatively fixed contact 17 connectedthereto. A movable contact 18 is electrically connected to oppositeterminal portion 15 by means of a conductor 19 and the usual currenttransfer lingers 20. The separable contacts 17 and 18, which are shownlatched in closed position by latching mechanism 21, are suitably biasedtoward open position and are adapted to separate in response to trippingof the latch 22, which is biased toward latching position. Aninterrupting chamber about the contacts 17 and 18 is defined by acylindrical member 23 formed of insulating material and suitablysupported from a base 24. Circuit breakers of this general type are wellknown in the art.

The interrupting chamber is mounted within a weatherproof insulatingcolumn or housing supported from the base 24 and comprising generallycylindrical porcelain shells 25, 25. Interposed between these porcelainshells, or housing portions, is a bushing current transformer assemblyC. T. comprising a pair of current transformers having windings 26 and27, each of which is wound about a conventional annular or tubular coreC surrounding the cylindrical member 23. These windings are generallytoroidal in form and also surround the cylindrical member 23. Winding 26is partially enclosed by a metallic shielding casing 28 and winding 27by a similar metallic shielding casing 29. These casings 28 and 29 areinterconnected through the outer peripheral portion of a metallic plate30 which constitutes conducting structure extending radially between thetwo current transformers and connected between opposed flanges formed atthe outer periphery 32 of casings 28 and 29, the metallic plate andcasing means being grounded at the outer peripheral portion of theassembly, as indicated at 31. For reasons which will become apparent asthe description proceeds, the inner peripheral portions 33 and 34 of therespective casings 28 and 29 are insulated at 35, 36 from the innerperipheral portion of the ground connected plate 30, and the onlyelectrical connection between the casings and the plate is adjacent theouter peripheries 32 thereof. Suitable insulation 37, 38 of a well-knowncharacter insulates the windings 26 and 27 from the casings 28, 29 andfrom the ground connected plate 30. Mounted radially inwardly of thecurrent transformer windings 26 and 27 and of casings 28 and 29 is agenerally cylindrical conductive sleeve 39 which constitutes conductiveshielding structure and extends axially along the current transformerassembly on opposite sides of the plate 30 and for a major portion ofthe axial length of said assembly. More specifically, the shieldingsleeve 39 is encompassed by the casings 28 and 29 throughoutsubstantially their entire extent. This conductive shielding sleeve isconnected to the inner periphery of grounded plate 30, is insulated fromthe casing means 28, 29 at all points except for the connection throughplate 30 and cooperates with the other structure of the currenttransformer assembly to produce the novel results hereinafter described.Suitable gaskets 40 and 41 at opposite ends of the assembly assure aweatherproof bushing structure.

Inspection of this current transformer assembly C. T., as shown in thedrawing, makes it apparent that the primary circuit of the currenttransformer secondary windings 26 and 27 normally comprises conductor 18and accordingly, current flow through conductor 18 normally induces acurrent flow in both secondary windings 26 and 27. Effective current owthrough conducting sleeve 39, since this sleeve 39 is encompassed by thecores C of the current transformer assembly, would similarly induce acurrent flow in a secondary winding 26 or 27. Current flows through thissleeve 39 only under internal fault conditions, such as indicated, forexample, at U or V, but under such conditions the sleeve 39 would, ineffect, constitute a portion of the primary circuit for the currenttransformer assembly. To explain further, if effective current flowsthrough fault V to ground, a current would be induced in secondarywinding 27, or alternatively, if effective current ows through fault Uto ground, current would be induced in winding 26. Thus, it may bestated that the paths formed by elements 18 or 39 through the currenttransformer assembly C. T. are inductively linked with the secondarywinding structure. As for fault currents flowing to ground in the casingmeans 28, 29, as would take place under external fault conditions, suchas illustrated at T or Y, these fault currents would have substantiallyno magnetic eifect on the secondary windings and, hence, such faultcurrents are considered to flow to ground through a breakdown path whichis essentially independent of inductive linkage with the secondarywindings. The insulation provided at the inner peripheral portions 33and 34 of casings 28 and 29 prevents such external fault currents fromowing to ground through a path encompassed by the secondary windings.The significance of these features of the current transformer assemblyC. T. will appear more clearly as the description proceeds.

In order to protect the electrical system shown in the drawing,normally-open diierential relays 5@ and 51 are provided for selectivelyor collectively controlling the tripping of the Circuit breakersincluded within the system. For this purpose, differential relay 5t)includes contacts 52 which when closed, establish a tripping circuit,generally indicated at A, for the tripping means associated with circuitbreakers 8, 8a and 3. For example, closure of contacts 52 connects thetripping means 13 and 13a of circuit breakers 8 and Sa, respectively,across the source 14 and, similarly, connects a tripping means 45 ofcircuit breaker 3 across source14. In a corresponding manner,differential relay 51 includes contacts 53 which upon closure areadapted by means of a tripping circuit., generally shown at B, toconnect the tripping means 13b, 13e, and 46 across source 14', inresponse to which breakers 8b',y `8c, and 3 will be tripped` open. w

Energization of differential relay 50 is effected from a differentialcircuit including current transformer seicondary windings 26, 54 and 55,which are energized in accordance with `the current flowing in theirrespective primaries which, under normal conditions, are conductors 18,l4- and 5. The circuit including the` transformer windings 26, 54, 55 isconstructed in a well-known manner so that under normal conditions, thatis, when there is no fault in the power circuit disposed between thetransformer secondaries, no effective current will flow in the windingof differential relay 50. The relay 50 is energized in accordance withthe difference between effective current flowing in the primaryconductors 18, 39` of winding 26 and the sum of the currents flowing inthe primaries 4 and 5 of windings 54 and 55, respectively. The operationof differential relay arrangements of this general type iswell-understood in the art, and it is apparent that other well-knowndifferential relay arrangements could equally well be used.

Energization of differential relay 51 is effected from a circuitincluding current transformer secondary windings 27, 56 and 57. Thewinding of relay 51 is connected in this circuit in the same manner asdescribed with respect to relay 50. Similarly, it will be understoodthat relay 51 will be energized in accordance with the differencebetween the effective vcurrent flowing in the primary Aconductors 1S, 39of winding 27 and the sum of the currents flowing in the primaries 6 and7 of windings 57 and 56, respectively.

It is apparent from the drawing that the current transformers of theassembly C. T. are mounted in overlapping relationship. That is, currenttransformer winding 26, which forms a part of the current transformermeans 26, 54, 55 protecting one portion of the system, extends into, oroverlaps into, the portion of the system protected by the secondtransformer means 27, 56, 57. This may be illustrated by the location ofcurrent transformer winding 26 in that portionof the primary powercircuit which includes the terminal 16 and which extends between thecurrent transformer windings 56, 57 and the current transformer winding27. More particularly, current transformer winding 26 is located betweenterminal 16 and current transformer winding 27. Similarly, the winding27 is located in that portion of the primary power circuit includingterminal and extending between windings 54, 55 and winding 26. Moreparticularly, winding 27 is located between terminal 15 and winding 26.

The tripping circuits A and B should desirably contain means fordeenergizing the tripping solenoids after said solenoids have effectedopening of their associated circuit breakers. To this end, contacts 48,Lilia, 4817 and 48e are provided in the tripping circuits of thebreakers 8, 8a, 8b and Sc, respectively. From the drawing, it will beobvious that these contacts will be opened in response to opening of anassociated breaker thereby deenergizing the associated trip means. ln alike manner, contacts 49a and 49h are provided in the tripping circuitsfor the breaker 3 and are adapted to be opened in response to trippingof breaker 3, thereby deenergizing trip means 45 or 46.

The operation of the above-described protective equipment will now bedescribed with the following considerations in mind. Faults in theinternal circuit of the circuit breaker are likely to be faults whichthe breaker itself cannot clear, and, therefore, all of the associatedbreakers must be relied upon to clear the fault so as to prevent currentfrom being fed from either of the bus sections into 6 thefault. Undersuch conditions, it is, therefore, necessary to open, in addition tofthe bus-tie breaker, the breakers in the bus sections on both sides ofthe bus-tie breaker. In contrast toy the requirements of this internalfault condition, in the event of a fault external to the bustie breaker,such as an insulation flashover to ground outside of the breakerhousing, in order to clear the fault the bus-tie breaker and only thosebreakers on the fault side of the bus-tie breaker need be opened-theelectric circuits on the unfaulted side of the breaker should desirablyremain operativ-ely connected to their bus section. l

The above requirements for collectively operatingall breakers in thecase of an internal fault and for selectively operating certain of saidbreakers in the case of an external fault necessitate that the currenttransformers associated with the circuit breaker be capable ofdistinguishing between internal and external faults. The novelconstruction of the current transformers of this in vention effectivelyAfulfills these requirements, as is demonstrated hereinafter.

Assume that a fault should occur at Y, i. e., an external fault orinsulation flashover on the bus-section 1 side of the bus-tie breaker.Under such conditions, it would be necessary to open only the bus-tiebreaker 3 and breakers 8 and 8a on the fault side of breaker 3, whileleaving the breakers Sb and 8c closed. If current be assumed to beflowing into the bus-tie breaker 3 from terminal 16 to terminal 15 4atthe instant the fault Y begins, it is apparent that there will be adifference in the current fowing through conductor 13 and the sum of thecurrents in corr ductors 4 and 5 since at least a portion of the currentflowing through conductor 18 is fed intothe fault. This difference incurrents will be detected by the current transformer windings 26, S4 and55, which will effect operation of differential relay 56, which in turneffects energization of trip means 13, 13a and 415- Thus, it will beseen that the breakers 8 and 3a and the bus-tie breaker 3 will beopened, as required. It will also be apparent that relay 51 will not beaffected by the fault Y., and hence, breakers 8b and 8c will remainclosed as desired. This is the case because, since the fault is at Y,thecurrent flo-wing through the conductor 18 will equal the sum of thecurrents flowing through conducting units 6 and 7. The fault current atY flows to ground at 31 through the outer casing 29 of the currenttransformer assembly, and since the breakdown path through casing 29 issubstantially independent of inductive linkage with the secondary winding 27, this fault current at Y has no effect on the energization ofsecondary winding 27.

lf Vthe fault be assumed to be located at the opposite external side ofthe bus-tie breaker, that is at T instead of Y, operation of relay 51will be effected, but relay Sil will remain deenergized, and as a resultonly circuit breakers 8b and Sc on the fault side of the bus-tie breakerand the bus-tie breaker 3 will be tripped open. The reason that relay 51will operate is, of course, because the primary current throughconductor 18 no longer equals the sum of currents in conductors 6 and 7,and so the sum of the secondary currents through windings 27, 56 and 57is no longer equal to zero. The reason that relay 50 remains inactive isbecause the fault current at T flows through the outer casing 28 of thecurrent transformer assembly, and so there is no effect on the secondarywinding 26 from this fault current at T. Thus, it is apparent that forexternal faults the protective equipment of this invention willselectively isolate the faulted portion of the electrical system.

With respect to internal faults, an essential characteristic of priorart differential protective arrangements is that the zone of theelectrical circuit between the secondary windings of the currenttransformers is coextensive with the internal circuit of the breakers.So long as it is practical to make this zone coextensive with theinternal circuit of the circuit breaker, the prior art arrangement coulddistinguish between external and internal faults. However, in certaintypes of circuit breakers, it is not practical because of cost or spacerequirements to mount the current transformer windings in locationswhich would produce the desired coextensiveness. ln such cases, if thecurrent transformers have metallic supporting parts grounded in themanner of the conventional current transformers, the currenttransformers could not distinguish between faults external to thebreaker and those in the internal circuit of the breaker but outside ofthe zone between the two current transformer windings.

This invention makes it possible to disregard the coextensiverelationship previously required, and hence to mount the overlappingcurrent transformers in a simple and inexpensive manner. By constructingthe current transformers in accordance with the invention, this simplemounting may be attained without affecting the ability of theoverlapping current transformers to distinguish between external andinternal faults. To this end, we have provided the current transformerassembly shown generally at C. T., the structural details of which havealready been described. Because of the particular structure of thebreaker with which this current transformer assembly C. T. isassociated, it is desirable to mount the assembly at one side of thefixed contact 17 and approximately midway between terminals and 16. Whenso mounted, the porcelain shells 25 and 25 encompass that portion of theinternal circuit which is outside of the zone extending between the twowindings of the current transformer assembly. In certain cases it isdesirable or necessary to mount the assembly nearer one or the other ofthe terminals. Additionally, in certain other cases, it is desirable tomount the individual current transformer secondaries in axially-spacedrelationship. The general design of current transformers constructed inaccordance with this invention will operate equally well for each ofthese Valternative mounting arrangements.

The characteristics of the currenttransformer assembly C. T. withrespect to the detection of internal faults will now be described. Ashas been pointed out, the primary circuit of the current transformersecondary windings 26 and 27 may comprise either the conductor 18 or theconducting shielding sleeve 39, and any effective current flow in eitherof these conductors 18 or 39 will induce a current flow in the secondarywinding through which said effective current flows. Assume now, forexample, that a fault occurs at V, i. e., at a location in the internal`lcircuit of the bus-tie breaker but outside of the zone of the circuitextending between the current transformer windings 26 and 27. Any faultto ground occurring within the circuit breaker housing will be directedto ground through conductive shielding members 39 and 30 since this isthe path of least breakdown strength for such faults. This generalconcept may be alternatively expressed by pointing out that forsubstantially all possible breakdown paths from said internal circuit toground, the grounded structure 39, which `is encompassed by thewindings, is interposed in the breakdown path between said internalcircuit and any other adjacent grounded structure. Thus, fault current Vwould be forced to follow a path to ground along said conductive member39 and between the current transformer windings 26 and 27. If it beassumed that current is owing from terminal 15 to terminal 16 of thecircuit breaker at the instant fault V occurs, it will be apparent thatthe current flowing in the primary conductor of the current transformerwinding 27 will not be equal to the sum of the currents in the primaryconductor of current trans former windings 56 and 57, since at least aportion of the current in the primary of winding 27 is diverted toground before reaching the transformer windings 56 and 57. Accordingly,relay 51 will be energized and the bus-tie breaker 3 together withbreakers 8b and 8c on one side of said bus-tie breaker will be tripped.It will also be apparent that when the fault occurs at V, the

current flowing in the primary of the current transformer winding 26will be unequal to the sum of the currents flowing in the primaries ofcurrent transformers 54 and 55, since at least a portion of the currentflowing from circuits 4 and 5 is diverted to ground through member 34)before reaching the primary of current transformer 26. Accordingly,relay will be energized to effect tripping of the breakers 8 and 8a onthe opposite side of the bus-tie breaker 3. Thus, it will be seen thatfor an internal fault V, the circuit breakers on both sides of thebus-tie breaker will be opened asdesired. Although, in the illustrativeexample, the current was assumed to be flowing from terminal 15 toterminal 16, the desired operation of the differential protective systemwill be obtained irrespective of the direction of current ow.

It will also be apparent that internal faults such as those located at Uor X, as shown in the drawing, will effect the operation of thedifferential protective system in substantially the same manner asdescribed with respect to the fault occurring at V, that is, thebreakers on both sides of the bus-tie breaker 3 will be tripped upon theoccurrence of such internal faults. For example, assume current flowfrom terminal 15 to 16 at the instant a fault occurs at U. Relay 51 willoperate due to the difference of full fault-produced current fromWinding 27 and zero fault-produced current from windings 56 and 57.Accordingly, breakers 3, 8b, and 8c will be tripped open. Assuming stilla fault at U and current flow from 15 to 16, relay 50 will `operate dueto the difference of `full fault-produced current from windings 54 and55 and zero fault-produced current from winding 26. The reason that nocurrent is induced in winding 26 under such conditions is that theprimary current for winding 26 flows from terminal 15 to U and thendoubles back through the shielding path 39, 30 to ground, thus insubstance cancelling out the effect of the primary current flow fromterminal 15 to fault U. Thus, since relay 50, under the assumedconditions, operates in response to this Zero current flow in winding26, the breakers 8 and 8a are tripped open. So, in summary, it may beseen that in response to a fault at U the breakers 8, 8a, 8b, and 8c atboth sides of the bus-tie breaker will be opened as desired. Similarly,for a fault at X instead of U these breakers 8, 8a, 8b and 8c at bothsides of the bus-tie breaker will be opened. For example, for a fault atX if current be assumed as flowing from terminal 15 to 16, winding 26will receive zero fault-produced current and windings 54, 55 willreceive full fault-produced current, thus causing operation of relay 50.Assuming still current ow from terminal 15 to 16, relay 51 will also beoperated because winding 27 receives full fault-produced current sincethe fault-produced current flows through the primary 18 of winding 27,however, windings 56 and 57 receive Zero fault-produced current sincethe fault current is diverted to ground at X before reaching themagnetic circuits of windings 56 and 57. Thus, the sum of the currentsthrough windings 27, 56 and 57 is no longer equal to zero and,accordingly, operation of relay 51 is effected. Thus, both relays 50 and51 are operated when an internal fault occurs at X, thereby opening thebreakers at both sides of the bus-tie breaker, as desired.

The above-described mode of operation illustrates that the currenttransformers of this invention are capable of accurately distinguishingbetween faults in any part of the internal circuit of the circuitbreaker and those faults external to said internal circuit, whereby toselectively or collectively control the associated protective equipmentdepending upon the location of the fault, as is desired.

While we have shown our invention as applied to a pair of currenttransformers, in some installations it is necessary to use differentialprotection on only one of the bus sections. In such installations, asingle current transformer located in the circuit of the bus-tie breakerand constructed in accordance with our invention would provide thedesired protection.

While there has been shown and described a particular embodiment of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andthat it is intended by the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A current transformer assembly adapted to be mounted about a primaryconductor, a pair of juxtaposed secondary windings having inner andouter peripheral portions, conductive casing means disposed about saidouter peripheral portion, conducting means extending from said innerperipheral portion between said windings and into an electricalconnection with said casing means adjacent said outer peripheralportion, said assembly being grounded at said outer peripheral portion,and means insulating said casing means from said conducting means at alllocations except at the outer peripheral portion of the assemblyelectrically between said windings for causing current in saidconducting means owing between said inner and outer peripheral portionsto flow to ground only in a path between said windings, and for causingcurrent owing to ground in said casing means to be essentiallymagnetically independent of said windings.

2. A current transformer assembly comprising a primary conductor, a pairof secondary windings positioned about said primary conductor, saidwindings having inner and outer peripheral portions, grounded conductivecasing means mounted about the outer peripheral portion of saidwindings, conductive shielding structure insulatingly interposed betweenthe inner peripheral portion of said windings and said primary conductorand encompassed by said windings, conducting means connecting saidshielding structure to ground and extending to ground by a path disposedelectrically between said windings and providing the sole conductivepath between said shielding structure and ground, and means insulatingsaid casing means from said shielding structure which is encompassed bysaid windings except for an electrical connection provided by saidconducting means which extends from said shielding structure to groundelectrically between said windings.

3. The assembly of claim 2 in which said shielding structure is ofgenerally tubular coniguration and extends throughout a major portion ofthe axial length of said assembly as measured along the length of saidprimary conductor.

4. A current transformer assembly adapted to be mounted about a primaryconductor, a pair of juxtaposed secondary windings having inner andouter peripheral portions, conductive casing means disposed about saidouter peripheral portion, conducting means extending from said innerperipheral portion between said windings and into an electricalconnection with said casing means adjacent said outer peripheralportion, said assembly being effectively grounded only at said outerperipheral portion, insulation separating said conducting means and saidcasing means at all points except at said connection electricallybetween said windings so that current owing in said casing means willflow to ground only in a path essentially independent of inductivelinkage with said windings and current owing in said conducting meanswill ilow to ground by a path inductively linked with one of saidwindings and located electrically between said windings.

5. In a eurent transformer assembly, a primary conductor, a pair ofspaced-apart generally tubular magnetic cores mounted about said primaryconductor, a pair of current transformer secondary windings, each ofwhich is interlinked with a different magnetic core, conductive casingmeans disposed about the exterior of said windings and connected toground, conducting means extending between said windings andelectrically interconnected to said casing means, and insulation meansdisposed so as to require currents in said casing means to ow to groundin a path external to the magnetic cores of said windings and so as torequire currents in said conducting means to flow to ground in a pathencompassed by the magnetic core of one of said windings.

6. The assembly of claim 5 being further characterized by saidconducting means comprising conducting structure mounted radially inwardof at least one of said cores.

7. The assembly of claim 6 in which said conducting structure extendsfor a major portion of the axial length of said assembly and isencompassed by said windings.

8. In a current transformer assembly, a primary conductor, a generallytubular magnetic core mounted about the conductor, a secondary windinginterlinked to said core, a conductive sleeve interposed between saidprimary conductor and said secondary winding and insulated from saidprimary conductor, a grounded conductive casing `mounted about the outerperiphery of the secondary winding, said casing encompassing saidconductive sleeve and mounted throughout substantially its entire extentradially outside of said sleeve, conducting means located solely at oneelectrical side of said secondary winding and forming the soleelectrical connection between said sleeve and said casing, andinsulating means for forcing all currents owing in said casing to iiowto ground by a path bypassing said sleeve, said insulating means forcingall currents ilowing through said sleeve to ilow to ground through saidsole electrical connection.

References Cited in the le of this patent UNITED STATES PATENTS 767,503Scott Aug. 16, 1904 2,327,774 Dickinson Aug. 24, 1943 2,549,426 ClarkApr. 17, 1951 FOREIGN PATENTS 487,651 Great Britain June 23, 1938

